1. Field of the Invention
The present invention relates to a semiconductor device provided with a function to control a current supplied to a load by a transistor, particularly relates to a display device including a pixel formed of a current drive type light emitting element in which luminance varies according to a current, and a signal line driver circuit thereof. In addition, the invention relates to a driving method of the display device, and to an electronic apparatus having the display device in a display portion.
2. Description of the Related Art
In recent years, a so-called self-luminous display device in which a pixel is formed of a light emitting element such as a light emitting diode (LED) has attracted attention. As a light emitting element used for such a self-luminous display device, an organic light emitting diode (an OLED), an organic EL element, an electroluminescence (EL) element, or the like have been attracted attention and have been used for an EL display or the like. A light emitting element such as an OLED is a self-luminous light emitting element, therefore, the light emitting element has an advantage such that the visibility of a pixel is high, a back light is unnecessary, and a response speed is fast compared to a liquid crystal display. Note that the luminance of a light emitting element is controlled by a current value flowing therethrough.
As a driving method to express a gradation of such a display device, there are a digital method and an analog method. The digital method turns on/off a light emitting element by a digital control to express a gradation. In the case of the digital method, there are only two states of light emission and non-light emission, therefore, only two gradations can be expressed. Thus, combining another method, many gradations are performed. As a method of many gradations, a time gradation method is often used. The digital time gradation method is superior to uniformity of a luminance in each pixel while it is required to increase frequency and power consumption increases. On the other hand, in the case of the analog method, the light intensity of a light emitting element is controlled in an analog manner or light emitting time of a light emitting element is controlled in an analog manner. The analog method of controlling light intensity is easy to be affected by characteristic variations of a thin film transistor (hereinafter also referred to as a TFT) in each pixel and variations are also generated in light emission in each pixel. On the other hand, described in a Non-Patent Document 1 is a display device of an analog time gradation method in which light emitting time is controlled in an analog manner and uniformity of light emission in each pixel is superior (See Non-Patent Document 1: SID 04 DIGEST p. 1394 to p. 1397).
A pixel of the display device described in Non-Patent Document 1 comprises an inverter formed of a light emitting element and a transistor for driving the light emitting element. A gate terminal of the driving transistor is an input terminal of the inverter and a drain terminal of the driving transistor is an output terminal of the inverter. Then, an output of the inverter is inputted to an anode of the light emitting element. When a video signal voltage is written to the pixel, the inverter is set in the middle of on and off. Then, a triangle wave voltage is inputted to the pixel to control the output of the inverter in a light emitting period. That is, the output of the inverter which is a potential inputted to the anode of the light emitting element is controlled, thereby controlling light emission/non-light emission of the light emitting element.
Here, a resistance load inverter is shown in FIG. 10B and inverter transfer characteristics of the resistance load inverter are shown in FIG. 10A. The abscissa in FIG. 10A indicates an input potential Vin into an input terminal of the resistance load inverter and the ordinate indicates an output potential Vout from an output terminal of the resistance load inverter. The resistance load inverter includes a transistor and a resistor, and a high power source potential Vdd is inputted to a source terminal of the transistor while a drain terminal thereof is connected to one terminal of the resistor. In addition, a low power source potential Vss is inputted to the other terminal of the resistor. Note that, here, Vss=0 V. A gate terminal of the transistor is the input terminal of the resistance load inverter while the drain terminal of the transistor is the output terminal of the resistance load inverter.
A curve 1002 shown in FIG. 10A shows inverter transfer characteristics of a resistance load inverter, a curve 1001 shows inverter transfer characteristics of a resistance load inverter in the case where a current supply capacity of the transistor in the inverter is high, and a curve 1003 shows inverter transfer characteristics of a resistance load inverter in the case where a current supply capacity of the transistor is low.
That is, when an input potential is sufficiently high and the transistor is in an off-state, a potential of the output terminal of the resistance load inverter becomes a potential of 0 V, while when the transistor is sufficiently in an on-state, the potential of the output terminal of the resistance load inverter becomes Vdd.
Here, the output Vout of the resistance load inverter is expressed below by using the power source potential Vdd, a resistance R of the resistor, and a source-drain current Id of the transistor.Vout=R×Id 
Further, the source-drain current Id of the transistor is expressed below when an operation is in a saturation region. Note that μ is a carrier mobility of the transistor, Cox is a capacitance of a gate insulating film, W/L is a ratio of a channel width W and a channel length L of the transistor, and Vth is a threshold voltage of the transistor.
  Id  =            1      2        ⨯    μ    ⨯    Cox    ⨯          W      L        ⨯                  (                  Vdd          -          Vin          -                                  Vth                                      )            2      
Therefore, the current supply capacity of the transistor varies in accordance with the value of μ, Cox, W/L, Vth, and the like. Accordingly, the inverter transfer characteristics of the resistance load inverter vary in accordance with variations of these values of the transistor.
Such variations of the inverter transfer characteristics of the resistance load inverter also occur in the case of using a light emitting element as a resistor. Then, even in the display device having the pixel described in Non-Patent Document 1, there is a pixel of the transfer characteristics of the resistance load inverter such as the curve 1001, the curve 1002, or the curve 1003. Accordingly, time from the transistor is turned on in the saturation region to the transistor is turned off and an output potential of the resistance load inverter becomes Vx, as well as time from a portion between the input terminal and the output terminal becomes conductive to input potentials Vinv1, Vinv2, and Vinv3 of the resistance load inverter which are offset-cancelled become input potentials Va1, Va2, and Va3 respectively at which the output potential of the resistance load inverter becomes Vx, varies in each pixel different in the transfer characteristics of the resistance load inverters.
Therefore, in a display device of a driving method for expressing a gradation in an analog time, even the same gradation display varies between pixels and clear display cannot be performed.
Further, there are problems in that the number of transistors or wires in a pixel is large, an aperture ratio decreases, and the like in conventional configurations. In the case where similar brightness is obtained in a pixel with a high aperture ratio and a pixel with a low aperture ratio, the pixel with a low aperture ratio is required to increase its luminance of the light emitting element more than the pixel with a high aperture ratio. Therefore, in the pixel with a low aperture ratio, deterioration of the light emitting element proceeds faster. In addition, the power consumption is also increased since the luminance is increased.
Moreover, when the number of transistors or wires in a pixel increases, yield also tends to decrease and cost of a display panel rises.